A light-emitting diode (LED) is widely known as a semiconductor light-emitting apparatus or a semiconductor light source which can emit light having a peak wavelength in a specific region of an optical spectrum. Typically, LEDs are used as light sources for illuminators, signs, in-vehicle headlamps, and displays. A light-emitting device emitting white light, in which an LED chip emitting blue light and a YAG (yttrium aluminum garnet) phosphor that converts blue light into yellow are combined, is known as a light-emitting device using an LED and a phosphor. The YAG phosphor is arranged, around the LED chip, as a wavelength conversion light-emitting layer dispersed in an epoxy resin or a silicone resin. In addition to the wavelength conversion light-emitting layer dispersed in the resin, a ceramic layer including a phosphor, or a wavelength conversion light-emitting layer (light-emitting ceramic layer) in which a phosphor is dispersed in ceramic and which consists of an inorganic material is described as an example (Patent Document 1).
In recent years, many novel substances related to nitrides including ternary or higher order elements have been produced. In particular, phosphor materials with excellent properties have been recently developed in multinary nitrides and oxynitrides based on silicon nitride, and have been used in wavelength conversion light-emitting layers. These phosphor materials are known to be excited to emit yellow or red light by blue LEDs or near ultraviolet LEDs, and exhibit high luminance, high conversion efficacy, and in addition, the excellent dependence of luminous efficacy on temperature, in comparison with oxide-based phosphors (Patent Document 2).
Conventionally, wavelength conversion light-emitting layers dispersed in organic binders such as epoxy resins and silicone resins have had insufficient durability, heat resistance, and emission intensity. Therefore, a method for producing a wavelength conversion light-emitting layer (light-emitting ceramic layer) consisting of an inorganic material has been researched as described as an example in Patent Document 1 in order to obtain a wavelength conversion light-emitting layer superior in durability and heat resistance.
Patent Document 3 describes, as an example, phosphor ceramic in which YAG:Ce phosphor particles are dispersed in an inorganic binder including any one of calcium fluoride, strontium fluoride, and lanthanum fluoride or including calcium fluoride and strontium fluoride.
In Patent Document 4, a wavelength conversion light-emitting layer consisting of an inorganic material is produced with a combination of an Y3(Al, Ga)5O12:Ce oxide phosphor or a Lu3Al5O12:Ce oxide phosphor with a CaSiAlN3:Eu (“CASN”) nitride phosphor by melting a glass powder having a glass transition point of 200° C. or more by using a discharge plasma sintering method.